Muscles in both humans and Drosophila are composed of multinucleate myofibers that are generated by the fusion of myoblasts. However, the events and molecules that regulate myoblast fusion are not well understood. Our long-term goal is to understand myoblast fusion, and in particular, the regulation of the number of fusion events required to create a muscle fiber. The objective of this proposal is to determine the critical cellular and molecular mechanisms controlling myoblast fusion in the model organism, Drosophila melanogaster. The conservation of genes and mechanisms in muscle development between Drosophila and mammals allows us to use the simpler Drosophila system to make relevant discoveries for treatments of human muscular diseases and of muscle wasting due to aging and chemotherapies. Our central hypothesis in this proposal is that specific cytoskeletal rearrangements are critical for myoblast fusion. Guided by our strong preliminary data, this hypothesis will be tested in three specific aims: (1) Identify the critical cytoskeletal rearrangements that underlie myoblast fusion;(2) Determine the requirement of known fusion genes in regulating specific cytoskeletal behaviors underlying myoblast fusion;and (3) Identify the role of new fusion genes in regulating the specific cytoskeletal rearrangements critical for myoblast fusion. Under the first aim, we have developed novel imaging techniques to identify cytoskeletal rearrangements during myoblast fusion in living and fixed embryos. Our preliminary data has pinpointed several such important cytoskeletal rearrangements. Under the second aim, we will test the impact of known fusion genes in relationship to these cytoskeletal rearrangements. Already we can link specific genes'activities to specific cytoskeletal rearrangements. Under the third aim we will investigate the mechanisms underlying the cytoskeletal changes during fusion by examining novel genes that we have identified. Our work is significant because it expected to reveal the cellular and molecular mechanisms underlying cell-cell fusion. The proposed research is relevant to public health because once the molecular players and the cellular targets of their action are identified or understood, therapies designed to regulate myoblast fusion can be developed to promote fusion for the treatment of muscle wasting due to aging or disease.